Direct engraving of printing blocks such as flexographic plate or gravure cylinders requires carving three-dimensional (3D) images on plate material directly with a laser system. This is significantly different from two dimensional imaging techniques which require post processing steps to produce the three-dimensional features.
This difference introduces several challenges for a laser imaging system for flexographic plates: the laser system must have sufficient power to ablate the material, and the laser spot should be small enough to achieve the fine detail required in quality printing. Although high power, high laser density does not necessary conflict with the laser focusability, from a practical perspective, these lasers have a high cost per watt ratio.
FIG. 1 illustrates a flexographic printing plate. As illustrated in FIG. 1, because the flexographic plate is pressed directly on the printed media such as paper, packaging material, etc., the areas 10 which transfer ink to the printed media need to be elevated from blank areas 11 which do not transfer ink. Typically the ink transfer areas 10 require engraving at the depth of 70 microns, whereas non ink transfer areas 11 will need to be engraved to the depth of 500-700 microns. The required depth of the blank areas is such that when the flexo plate is pressed against another surface, for instance the ink carrying agent, e.g., anilox roll, and subsequently on the printed media, these blank areas will be kept out of contact with other surfaces.
In printing, a plate is pressed firmly against another surface, such as packaging. Because a flexo plate is deformable, imaging features separated by large blank areas will be deformed more strongly. As a result, large blank areas will be pushed towards the contact surface more strongly than small blank areas. This is depicted schematically in FIG. 2, where the applied pressure 20 pushes large blank area 21 more strongly than small blank area 22 towards press contact surface 23. Therefore, large blank areas must maintain greater depth than small areas, sufficient to prevent contact with the contact surface. To summarize the above, it follows that fine blank areas can be engraved by the laser system to a shallower relief than that required for large areas.
In graphic arts large, blank areas are identified to produce large non ink areas when printed. The regions with small blank areas correspond to fine detail areas when printed. The energy required to engrave a flexographic plate area, equivalent of 1 square centimeter in a depth of 1 micron is about 0.45 Joule. Typical electrical power required for removing flexographic plate non ink transfer areas 11 will be on the order of 1000 of watts whereas, ink transfer areas 10 will need about 200 watts. The ratio of the above electrical consumption correlates to the depths of ink transfer areas 10 and non ink transfer areas 11, 70 micron for ink transfer areas 10 and 500-700 microns for areas 11.
The engraving process of flexographic plate production generates large amount of debris. This creates a challenge to remove the generated debris, consisting of small residual particles and emission of gas, in an effective manner. This adds extra complexity and cost to the process. The above problems are solved by the present invention.